1. Field of the Invention
This invention relates to an optical space transmitter for bidirectionally transmitting information between remote sites by way of light beams using a common optical axis for both transmitted beams and received beams.
2. Related Background Art
Generally, optical space transmitters/receivers for transmitting information to a remote site and receiving information from the remote site by means of optical beams have a disadvantage that the optical axis of the beam transmitted from the partner transmitter for signal transmission/reception and that of the light receiving section of the receiver can become displaced from each other for a variety of reasons, including natural phenomena such as winds and sun beams and man-made problems, and end up with totally disrupted communications, in the worst case. Therefore, an optical space transmitter is normally provided with an angle correction feature for correcting the angular displacement of the beam for signal transmission/reception.
FIG. 1 of the accompanying drawings schematically illustrates a principal part of a conventional optical space transmitter/receiver. Referring to FIG. 1, the signal to be transmitted that is inputted through input terminal 14 is sent to light source 1 (electrooptic converter) adapted to emit a beam that is modulated according to the signal. The light beam emitted from the light source 1 is then sent out by way of lens 2, beam splitter 3, tracking mirror 4 and lenses 5 and 6. On the other hand, the beam transmitted from the partner transmitter is received by way of the lenses 6 and 5 and led to the light receiving section by way of the tracking mirror 4 and the beam splitter 3 and then divided into two beams by half mirror 7 to proceed in two directions. One of the beams is reflected by the half mirror 7 and collected, by way of lens 10, by first photodetector (main signal receiving section) 8 that converts the beam into a reception signal, which is then taken out through output terminal 15. The other beam is transmitted through the half mirror 7 and collected, by way of lens 11, by second photodetector (angle error detector) 9.
The second photodetector (angle error detector) 9 detects the angular displacement of the optical axis of the beam transmitted from the partner transmitter and the optical axis of the angle error detector that is typically indicated by the optical axis of the lens 11. Then, optical axis angular regulation drive control section 12 controls actuator 13 on the basis of the information on the angular displacement and automatically corrects the angular displacement by regulating the angle of the tracking mirror 4.
For the second photodetector (angle error detector) 9 to detect the angular displacement and direct the transmission light beam accurately to the partner device, it is necessary to make the optical axis L1 of the transmission light beam outputted from the light source (transmitting section) 1 of the transmitter/receiver and the optical axis L2 of the angle error detector (the optical axis of the lens 11) agree with each other within the device in advance. In order to make the two optical axes L1 and L2 agree with each other, it is necessary to make the optical axes L1 and L2 to follow a same light path between the tracking mirror 4 and the beam splitter 3. In operation, the depicted device constantly detects the angular displacement between the optical axis L3 of the beam transmitted from the partner device and received by the depicted device and the optical axis L2 of the angle error detector of the depicted device, that is the optical axis L1 of the beam transmitted from the present device, and, if necessary, correct it to eliminate any relative displacement of the two optical axes.
However, with the above described known transmitter/receiver, particularly when the device is arranged outdoors and rises to almost about 40° C. during the day time in the Summer in Japan, raising the internal temperature of the device even further, the optical system including the lens barrel can thermally expand to produce a relative displacement between the optical axis of the beam to be transmitted and that of the received beam.
Particularly, when ambient temperature is too high or too low, the optical axis L1 of the beam to be transmitted from the transmitter of the device and the optical axis of the angle error detector (the optical axis of the lens 11) L2 are displaced, if only slightly, from each other due to the expanded or compressed optical system including the lens barrel. Therefore, the optical axis L1 of the beam to be transmitted from the depicted device and the optical axis L3 of the received light beam transmitted from the partner device do not agree with each other even if the angular displacement of the optical axis L3 of the received light beam transmitted from the partner device and the optical axis L2 of the angle error detector (the optical axis of the lens 11) is detected and corrected. Then, it is not possible to reliably transmit a beam to the partner device.
Additionally, when the external factors including winds and sun beams are most unfriendly, the light beam transmitted from the device A can partly go astray from the partner device B as shown in FIG. 2 and end up with a total inability of communication. A countermeasure taken for remedying this problem is the use of a large beam diameter for the purpose of accommodating the displacement of the possible optical axis so that the optical axis of the beam transmitted from the own device A may not be totally moved away from the partner device B. However, if ambient temperature in operation is in the temperature level used for regulating the optical axis and close to room temperature, it is not necessary to use a large beam diameter because the displacement of the optical axis is, if any, very small. Since the quantity of light the partner device B receives per unit time decreases by an amount inversely proportional to the square of the increase in the beam diameter, the allowable attenuation of the transmission path is disadvantageously reduced most of the time, except the time when ambient temperature is extremely high and the time when it is extremely low.
Additionally, the expansion/compression of the optical system due to temperature changes entails, beside the above optical axis displacement, a change in the distance between the transmitting section and the lens to consequently displace the focal point of the optical system because the transmitting section is moved away from the stretch of the focal length of the lens by the thermal expansion of the lens barrel to consequently change the angle of expansion of the beam transmitted from the device. Since this change narrows the angle of expansion at high temperature, it goes far below the desired angle when the external factors including winds and sun beams are most unfriendly so that the light beam from the device can be moved away from the partner device to end up with a total inability of communication. If, to the contrary, the angle of expansion is too wide, the quantity of light the partner device receives per unit time is reduced too much, which in turn reduces the allowable attenuation of the transmission path.